Immune System

Cells of the Immune System

The immune system is a host defense system comprising many biological structures and processes within an organism that protects against disease. To function properly, an immune system must detect a wide variety of agents, known as pathogens, from viruses to parasitic worms, and distinguish them from the organism’s own healthy tissue. The white blood cells of the immune system are macrophages, neutrophils, mast cells, natural killer cells, dendritic cells. Macrophages are leukocytes that phagocytose pathogen and then act as antigen presenting cell. Neutrophils are polymorphonuclear leukocytes that phagocytose pathogen and destroys it. Mast cells are leukocytes that release histamine during an allergic response, bringing about inflammation. Natural killer cells kills infected/abnormal cells and dendritic cells are the best antigen presenting cells. T-lymphocytes undergo maturity in the thymus. Cytotoxic T cells recognize antigen on infected cells, and signal for apoptosis (cell death). Helper T cells recognize antigen on antigen-presenting cells, and signal for activation of B cells, T cells, and macrophages. B-lymphocytes and plasma cells undergo maturity in the bone marrow. B cells form plasma cells and memory cells when exposed to antigen. Plasma cells secrete antibody and memory cells stick around in case the same antigen attacks in the future.

Innate vs Adaptive Immunity

All blood cells arise from stem cells in the bone marrow. B lymphocytes differentiate in the bone marrow. The spleen provides a site for white blood cells to reside and proliferate. It also removes pathogens from blood as well as old red blood cells and platelets. The lymph nodes provide a site for white blood cells to reside and proliferate and it removes pathogens from lymph. Residing lymphocytes monitor lymph for foreign antigens, and initiate an immune response when exposed to foreign antigens. Innate immunity is the first line of defense and it kills anything that doesn’t look right within the host body. It is not specific to a particular pathogen / antigen. White blood cells are more active at higher temperature and inflammation calls white blood cells to the site of infection by sending out chemical signals and making capillaries more permeable. Adaptive immunity is highly specific for a particular pathogen / antigen unlike innate immunity. This is the immunity where antigen presenting cells present foreign antigen on their surface leading to destruction of that specific antigen carrying pathogen. B cells produce antibodies which bind to antigens to prevent pathogen from adhering to host cells. Opsonization makes it easier for phagocytosis. It complements activation by killing infected cell by punching holes in their cell membrane.

Memory cells are made that are much more efficient in proliferating and making antibodies in case the same infection strikes in the future. Memory cells allow the body to mount a greater, and more sustained response against the same pathogen during secondary response. A secondary response does not need T cell activation resulting in the body fighting off the infection much quicker.

Antigens

The concept of antigen and antibody is similar to a lock and key model, where a specific antigen (key) fits a specific antibody only (lock). Antibody has the appearance of a Y, with the tips of the Y being capable of binding antigen. The tips of the fork are called hypervariable regions because they are unique to each antigen-specific antibody. The antibody consists of 2 light chains and 2 heavy chains linked together by disulfide bonds.

 

By DigitalShuttermonkey (Recreated jpg originally uploaded by Muntasir Alam) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
The mechanism of stimulation by antigen is initiated by antigen presentation. Pathogen enters antigen-presenting-cell (APC) and pieces of the pathogen gets displayed at the surface of APCs. T cell receptors recognize the presented antigen, and activates various immune responses. In one instance, the extracellular pathogen enters the body and macrophages engulf the pathogen. Pieces of the pathogen becomes the antigen and gets presented at the macrophage’s cell surface. Helper T cells recognize the presented antigen, and activates macrophages to destroy pathogen. Helper T cells also activate B cells to produce antibodies against the pathogen. For a intracellular pathogen, pathogen invades host cell and pieces of the pathogen gets presented on the host cell surface. Cytotoxic T cells recognize the presented antigen, and signals the infected cell to self-destruct.

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